U.S. patent application number 13/073525 was filed with the patent office on 2011-07-14 for method and apparatus for accommodating differrent clock frequencies in an ethernet passive optical network.
Invention is credited to Edward W. BOYD, Ryan E. Hirth, Hung Nguyen.
Application Number | 20110170868 13/073525 |
Document ID | / |
Family ID | 37596029 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110170868 |
Kind Code |
A1 |
BOYD; Edward W. ; et
al. |
July 14, 2011 |
Method and Apparatus for Accommodating Differrent Clock Frequencies
in an Ethernet Passive Optical Network
Abstract
One embodiment of the present invention provides a system that
accommodates different clock frequencies in an Ethernet passive
optical network (EPON). The system receives a signal from an
optical line terminal (OLT) at an optical network unit (ONU) and
derives an OLT clock. The system also maintains a local clock. The
system further receives from the OLT an assignment for an upstream
transmission window, during which the ONU can transmit an upstream
data burst to the OLT based on the local clock. The system adjusts
the number of bits of the data burst without affecting the payload
data carried in the data burst, thereby allowing the data burst to
fit properly within the upstream transmission window and
compensating for frequency differences between the local clock and
the OLT clock. The system transmits the data burst based on the
local clock in the upstream transmission window.
Inventors: |
BOYD; Edward W.; (Petaluma,
CA) ; Hirth; Ryan E.; (Windsor, CA) ; Nguyen;
Hung; (Rohnert Park, CA) |
Family ID: |
37596029 |
Appl. No.: |
13/073525 |
Filed: |
March 28, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11478492 |
Jun 28, 2006 |
7929576 |
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13073525 |
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60695795 |
Jun 29, 2005 |
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Current U.S.
Class: |
398/58 |
Current CPC
Class: |
H04J 3/0682 20130101;
H04J 3/0652 20130101; H04Q 11/0067 20130101; H04J 3/07 20130101;
H04J 3/1694 20130101; H04J 3/0638 20130101; H04Q 2011/0079
20130101 |
Class at
Publication: |
398/58 |
International
Class: |
H04B 10/20 20060101
H04B010/20 |
Claims
1. A method for accommodating different clock frequencies in an
Ethernet passive optical network (EPON), the EPON including an
optical line terminal (OLT) and one or more optical network units
(ONU's), comprising: (a) comparing, by an ONU from among the ONU's,
an ONU clock and an OLT clock to determine a difference between the
ONU clock and the OLT clock; (b) requesting, by the ONU, an
upstream transmission window to transmit an upstream data burst
that compensates for the difference between the ONU clock and the
OLT clock; and (c) transmitting, by the ONU, the upstream data
burst during the upstream transmission window.
2. The method of claim 1, wherein step (a) comprises: (a)(i)
receiving a signal from the OLT; (a)(ii) deriving the OLT clock
from the signal; and (a)(iii) comparing the ONU clock and the OLT
clock to determine the difference between the ONU clock and the OLT
clock.
3. The method of claim 1, wherein step (a) comprises: (a) comparing
the ONU clock and the OLT clock to determine whether the ONU clock
is faster or slower than the OLT clock.
4. The method of claim 1, wherein step (b) comprises: (b)(i)
determining a second upstream transmission window in terms of an
amount of data that is to be transmitted based on the OLT
clock.
5. The method of claim 4, wherein step (b) further comprises:
(b)(ii) requesting a larger upstream transmission window than is
necessary to transmit the upstream data burst using the second
upstream transmission window when the ONU clock is slower than the
OLT clock.
6. The method of claim 4, wherein step (b) further comprises:
(b)(ii) requesting a smaller upstream transmission window than is
necessary to transmit the upstream data burst using the second
upstream transmission window when the ONU clock is faster than the
frequency of the OLT clock.
7. The method of claim 1, wherein the upstream data burst is
characterized as including a first number bits, and wherein step
(b) comprises: (b)(i) requesting a upstream transmission window
that is necessary to transmit a second number of bits when the ONU
clock is slower than the OLT clock, the second number of bits being
greater than the first number of bits.
8. The method of claim 1, wherein the upstream data burst is
characterized as including a first number bits, and wherein step
(b) comprises: (b)(i) requesting a upstream transmission window
that is necessary to transmit a second number of bits when the ONU
clock is faster than the OLT clock, the second number of bits being
less than the first number of bits.
9. The method of claim 1, wherein step (b) comprises: (b)(i)
determining a size of a second upstream transmission window in
terms of an amount of data that is to be transmitted based on the
OLT clock; and (b)(i) requesting a size of the upstream
transmission window to be larger than the size of the second
upstream transmission window when the ONU clock is slower than the
OLT clock.
10. The method of claim 1, wherein step (b) comprises: (b)(i)
determining a size of a second upstream transmission window in
terms of an amount of data that is to be transmitted based on the
OLT clock; and (b)(i) requesting a size of the upstream
transmission window to be smaller than the size of the second
upstream transmission window when the ONU clock is faster than the
OLT clock.
11. An Ethernet passive optical network (EPON) for accommodating
for different clock frequencies, comprising: an optical network
unit (ONU), coupled to an optical line terminal (OLT), configured
to: (i) compare an ONU clock and an OLT clock to determine a
difference between the ONU clock and the OLT clock, (ii) request an
upstream transmission window to transmit an upstream data burst
that compensates for a difference between the ONU clock and the OLT
clock, and (iii) to transmit the upstream data burst during the
upstream transmission window.
12. The EPON of claim 11, wherein the ONU is further configured to
receive a signal from the OLT and to derive the OLT clock from the
signal.
13. The EPON of claim 11, wherein the ONU is configured to compare
the ONU clock and the OLT clock to determine whether the ONU clock
is faster or slower than the OLT clock
14. The EPON of claim 11, wherein the ONU is configured to
determine a second upstream transmission window in terms of an
amount of data that is to be transmitted based on the OLT
clock.
15. The EPON of claim 14, wherein the ONU is further configured to
request a larger upstream transmission window than is necessary to
transmit the upstream data burst using the second upstream
transmission window when the ONU clock is slower than the OLT
clock.
16. The EPON of claim 14, wherein the ONU is further configured to
request a smaller upstream transmission window than is necessary to
transmit the upstream data burst using the second upstream
transmission window when the ONU clock is faster than the frequency
of the OLT clock.
17. The EPON of claim 11, wherein the upstream data burst is
characterized as including a first number bits, and wherein the ONU
is further to request a upstream transmission window that is
necessary to transmit a second number of bits when the ONU clock is
slower than the OLT clock, the second number of bits being greater
than the first number of bits.
18. The EPON of claim 11, wherein the upstream data burst is
characterized as including a first number bits, and wherein the ONU
is further to request a upstream transmission window that is
necessary to transmit a second number of bits when the ONU clock is
faster than the OLT clock, the second number of bits being less
than the first number of bits.
19. The EPON of claim 11, wherein the ONU is configured to
determine a size of a second upstream transmission window in terms
of an amount of data that is to be transmitted based on the OLT
clock and to request a size of the upstream transmission window to
be larger than the size of the second upstream transmission window
when the ONU clock is slower than the OLT clock.
20. The EPON of claim 11, wherein the ONU is configured to
determine a size of a second upstream transmission window in terms
of an amount of data that is to be transmitted based on the OLT
clock and to request a size of the upstream transmission window to
be smaller than the size of the second upstream transmission window
when the ONU clock is faster than the OLT clock.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 11/478,492, filed on Jun. 28, 2006, which
claims the benefit of U.S. Provisional Patent Appl. No. 60/695,795
filed on Jun. 29, 2005. U.S. patent application Ser. No. 11/478,492
is incorporated herein by reference in its entirety.
BACKGROUND
[0002] 1. Field of Invention
[0003] The present invention relates to the design of Ethernet
passive optical networks. More specifically, the present invention
relates to a method and a system that accommodates different clock
frequencies between an optical line terminal and an optical network
unit in an Ethernet passive optical network.
[0004] 2. Related Art
[0005] In order to keep pace with increasing Internet traffic,
optical fibers and associated optical transmission equipment have
been widely deployed to substantially increase the capacity of
backbone networks. However, this increase in the capacity of
backbone networks has not been matched by a corresponding increase
in the capacity of access networks. Even with broadband solutions,
such as digital subscriber line (DSL) and cable modem (CM), the
limited bandwidth offered by current access networks creates a
severe bottleneck in delivering high bandwidth to end users.
[0006] Among the different technologies that are presently being
developed, Ethernet passive optical networks (EPONs) are one of the
best candidates for next-generation access networks. EPONs combine
ubiquitous Ethernet technology with inexpensive passive optics.
Hence, they offer the simplicity and scalability of Ethernet with
the cost-efficiency and high capacity of passive optics. In
particular, due to the high bandwidth of optical fibers, EPONs are
capable of accommodating broadband voice, data, and video traffic
simultaneously. Such integrated service is difficult to provide
with DSL or CM technology. Furthermore, EPONs are more suitable for
Internet Protocol (IP) traffic, because Ethernet frames can
directly encapsulate native IP packets with different sizes,
whereas ATM passive optical networks (APONs) use fixed-size ATM
cells and consequently require packet fragmentation and
reassembly.
[0007] Typically, EPONs are used in the "first mile" of the
network, which provides connectivity between the service provider's
central offices and business or residential subscribers. Logically,
the first mile is a point-to-multipoint network, with a central
office servicing a number of subscribers. A tree topology can be
used it an EPON, wherein one fiber couples the central office to a
passive optical splitter, which divides and distributes downstream
optical signals to subscribers and combines upstream optical
signals from subscribers (see FIG. 1).
[0008] Transmissions within an EPON are typically performed between
an optical line terminal (OLT) and optical networks units (ONUs)
(see FIG. 2). The OLT generally resides in the central office and
couples the optical access network to a metro backbone, which is
typically an external network belonging to an Internet Service
Provider (ISP) or a local exchange carrier. An ONU can be located
either at the curb or at an end-user location, and can provide
broadband voice, data, and video services. ONUs are typically
coupled to a one-by-N (1.times.N) passive optical coupler, where N
is the number of ONUs, and the passive optical coupler is typically
coupled to the OLT through a single optical link. (Note that one
may use a number of cascaded optical splitters/couplers.) This
configuration can significantly save the number of fibers and
amount of hardware required by EPONs.
[0009] Communications within an EPON can be divided into downstream
traffic (from OLT to ONUs) and upstream traffic (from ONUs to OLT).
In the downstream direction, because of the broadcast nature of the
1.times.N passive optical coupler, downstream data frames are
broadcast by the OLT to all ONUs and are selectively extracted by
their destination ONUs. In the upstream direction, the ONUs need to
share channel capacity and resources, because there is only one
link coupling the passive optical coupler with the OLT.
[0010] One challenge in designing an EPON is to mitigate the effect
of different clock frequencies used for transmission at the ONUs.
When an ONU's transmission clock frequency drifts away from the OLT
clock, the actual time consumed by an upstream burst from that ONU
may be longer or shorter than the size of the assigned transmission
window. This may lead to erroneous reception of the upstream bits
and potentially cause overlapping between two consecutive upstream
bursts from different ONUs.
[0011] Hence, what is needed is a method and a system for
accommodating different clock frequencies within an EPON so that an
upstream data burst remains within its assigned transmission
window.
SUMMARY OF THE INVENTION
[0012] One embodiment of the present invention provides a system
that accommodates different clock frequencies in an Ethernet
passive optical network (EPON), wherein the EPON includes an
optical line terminal (OLT) and one or more optical network units
(ONU's). During operation, the system receives a signal from the
OLT at the ONU and derives an OLT clock from the received signal.
The system also maintains a local clock. The system further
receives from the OLT an assignment for an upstream transmission
window, during which the ONU can transmit an upstream data burst to
the OLT based on the local clock. The system adjusts the number of
bits of the data burst without affecting the payload data carried
in the data burst, thereby allowing the data burst to fit properly
within the upstream transmission window and compensating for
frequency differences between the local clock and the OLT clock.
The system then transmits the data burst based on the local clock
in the upstream transmission window.
[0013] In a variation on this embodiment, adjusting the number of
bits of the data burst involves inserting one or more unused bits
into the data burst without affecting the payload data when the
transmission time of the data burst based on the local clock is
shorter than the transmission time based on the OLT clock by a
pre-determined threshold.
[0014] In a further variation, the predetermined threshold
corresponds to the transmission time of a character based on the
local clock or the OLT clock. Furthermore, inserting one or more
unused bits into the data burst involves inserting an idle
character into an inter-packet gap between two packets contained in
the data burst.
[0015] In a variation on this embodiment, adjusting the number of
bits of the data burst involves removing one or more unused bits
from the data burst without affecting the payload data when the
transmission time of the data burst based on the local clock is
longer than the transmission time based on the OLT clock by a
pre-determined threshold.
[0016] In a further variation, the pre-determined threshold
corresponds to the transmission time of a character based on the
local clock or the OLT clock. Furthermore, removing one or more
unused bits from the data burst involves removing an idle character
from an inter-packet gap between two packets contained in the data
burst.
[0017] A further embodiment of the present invention provides a
system that accommodates different clock frequencies in an EPON,
wherein the EPON includes an OLT and one or more ONU's. During
operation, the system receives a signal from the OLT at the ONU and
derives an OLT clock from the received signal. The system also
maintains a local clock. The system further compares the local
clock with the OLT clock. The system requests from the OLT an
upstream transmission window based on the comparison, wherein the
upstream transmission window can accommodate an upstream data burst
transmitted based on the local clock.
[0018] In a variation on this embodiment, the system determines the
duration of the upstream transmission window based on the size of
the data burst and the local clock. The system further determines
the size of the upstream transmission window in terms of amount of
data that can be transmitted based on the OLT clock during the
determined duration. Furthermore, requesting the upstream
transmission window involves communicating to the OLT the
determined size of the upstream transmission window in terms of
amount of data to be transmitted based on the OLT clock.
[0019] A further embodiment of the present invention provides a
system for accommodating different clock frequencies in an EPON,
the EPON including an OLT and one or more ONU's. The system
includes an OLT-clock derivation module configured: (i) to receive
a signal, (ii) to derive an OLT clock from the signal, and (iii) to
maintain a local clock to simulate an OLT clock. The system
additionally includes a receiving module configured to receive an
assignment for an upstream transmission window for transmission of
an upstream data burst based on the local clock. The system further
includes a determination module configured to determine a size of
the upstream transmission window in terms of an amount of data that
is to be transmitted based on the OLT clock. The system yet further
includes an adjustment module configured to adjust a number of bits
of the upstream data burst without affecting a payload data carried
in the upstream data burst by inserting or removing unused bits in
corresponding inter-packet gaps between packets contained in the
upstream data burst, thereby allowing the upstream data burst to
fit properly within the upstream transmission window and
compensating for frequency differences between the local clock and
the OLT clock. The system yet further includes a transmission
module configured to transmit the upstream data burst based on the
local clock during the upstream transmission window. In this
further embodiment, a number of bits inserted or removed is
determined by the difference between the size of the upstream
transmission window and a size of upstream data burst.
[0020] In a variation on this embodiment, the adjustment module
includes an insertion module configured to insert one or more
unused bits into the upstream data burst without affecting the
payload data when a transmission time of the upstream data burst
based on the local clock is shorter than a transmission time based
on the OLT clock by a pre-determined threshold.
[0021] In an additional variation on this embodiment, the
pre-determined threshold corresponds to a transmission time of a
character based on the local clock or the OLT clock. In this
additional variation, the insertion module is configured to insert
an idle character into the corresponding inter-packet gaps between
the packets contained in the upstream data burst.
[0022] In a further variation on this embodiment, the adjustment
module includes a removal module configured to remove one or more
unused bits from the upstream data burst without affecting the
payload data when a transmission time of the upstream data burst
based on the local clock is longer than a transmission time based
on the OLT clock by a pre-determined threshold.
[0023] In a yet further variation on this embodiment, the
pre-determined threshold corresponds to a transmission time of a
character based on the local clock or the OLT clock. In this yet
further variation, the removal module is configured to remove an
idle character into the corresponding inter-packet gaps between the
packets contained in the upstream data burst.
[0024] A further embodiment of the present invention provides a
system for accommodating different clock frequencies in an EPON,
the EPON including an OLT and one or more ONU's. The system
includes an OLT-clock derivation module configured (i) to receive a
signal, (ii) to derive an OLT clock from the signal, and (iii) to
maintain a local clock to simulate an OLT clock. The system
additionally includes a comparison module configured to compare the
local clock and the OLT clock. The system further includes a
request module configured to request an upstream transmission
window based on the comparison. In this further embodiment, the
upstream transmission window is configured to accommodate an
upstream data burst transmitted based on the local clock.
[0025] In a variation on this embodiment, the system further
includes a determination module configured to: (i) determine a
duration of the upstream transmission window based on a size of the
upstream data burst and the local clock, and (ii) to determine a
size of the upstream transmission window in terms of an amount of
data that is to be transmitted based on the OLT clock during the
duration of the upstream transmission window. In this variation,
the requesting module is configured to communicate the size of the
upstream transmission window in terms of the amount of data that is
to be transmitted based on the OLT clock.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
[0026] FIG. 1 illustrates an Ethernet passive optical network
wherein a central office and a number of subscribers are coupled
through optical fibers and an Ethernet passive optical splitter
(prior art).
[0027] FIG. 2 illustrates an EPON in normal operation mode (prior
art).
[0028] FIG. 3A illustrates an ONU generating a loop-back clock
frequency based on an OLT's downstream clock frequency and a
possible overlapping between two upstream bursts caused by
excessive clock-frequency drifting (prior art).
[0029] FIG. 3B illustrates two ONUs equipped with locally generated
clock frequencies in accordance with an embodiment of the present
invention.
[0030] FIG. 4 illustrates the process of inserting an idle
character into or removing an idle character from an inter-packet
gap (IPG) within a data burst transmitted from an ONU in accordance
with an embodiment of the present invention. The present invention
will now be described with reference to the accompanying drawings.
In the drawings, like reference numbers generally indicate
identical, functionally similar, and/or structurally similar
elements. The drawing in which an element first appears is
indicated by the leftmost digit(s) in the reference number.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The following description is presented to enable any person
skilled in the art to make and use the invention, and is provided
in the context of a particular application and its requirements.
Various modifications to the disclosed embodiments will be readily
apparent to those skilled in the art, and the general principles
defined herein may be applied to other embodiments and applications
without departing from the spirit and scope of the present
invention (e.g., general passive optical network (PON)
architectures). Thus, the present invention is not intended to be
limited to the embodiments shown, but is to be accorded the widest
scope consistent with the principles and features disclosed
herein.
[0032] The data structures and procedures described in this
detailed description are typically stored on a computer readable
storage medium, which may be any device or medium that can store
code and/or data for use by a computer system. This includes, but
is not limited to, application specific integrated circuits
(ASICs), field-programmable gate arrays (FPGAs), semiconductor
memories, magnetic and optical storage devices such as disk drives,
magnetic tape, CDs (compact discs), and DVDs (digital versatile
discs or digital video discs).
[0033] Passive Optical Network Topology
[0034] FIG. 1 illustrates a passive optical network, wherein a
central office and a number of subscribers are coupled together
through optical fibers and a passive optical splitter (prior art).
As shown in FIG. 1, a number of subscribers are coupled to a
central office 101 through optical fibers and a passive optical
splitter 102. Passive optical splitter 102 can be placed in the
vicinity of end-user locations, so that the initial fiber
deployment cost is minimized. Central office 101 can be coupled to
an external network 103, such as a metropolitan area network
operated by an Internet service provider (ISP). Note that although
FIG. 1 illustrates a tree topology, a PON can also be based on
other topologies, such as a ring or a bus.
[0035] Normal Operation Mode in EPON
[0036] FIG. 2 illustrates an EPON in normal operation mode (prior
art). To allow ONUs to join an EPON at arbitrary times, an EPON
typically has two modes of operation: a normal operation mode and a
discovery (initialization) mode. Normal operation mode accommodates
regular upstream data transmissions, where an OLT assigns
transmission opportunities to all initialized ONUs.
[0037] As shown in FIG. 2, in the downstream direction, OLT 201
broadcasts downstream data to ONU 1 (211), ONU 2 (212), and ONU 3
(213). While all ONUs may receive the same copy of downstream data,
each ONU selectively forwards only the data destined to itself to
its corresponding users, which are user 1 (221), user 2 (222), and
user 3 (223), respectively.
[0038] In the upstream direction, OLT 201 first schedules and
assigns transmission timeslots to each ONU according to the ONU's
service-level agreement. When not in its transmission timeslot, an
ONU typically buffers the data received from its user. When its
scheduled transmission timeslot arrives, an ONU transmits the
buffered user data within the assigned transmission window.
[0039] Since every ONU takes turns in transmitting upstream data
according to the OLT's scheduling, the upstream link's capacity can
be efficiently utilized. However, for the scheduling to work
properly, the OLT needs to discover and initialize a newly joined
ONU. During discovery, the OLT may collect information critical to
transmission scheduling, such as the ONU's round-trip time (RTT),
its media access control (MAC) address, its service-level
agreement, etc. (Note that in some cases service-level agreement
may already be known to the OLT.)
[0040] Generating Frequency for Upstream Transmission
[0041] It is of critical importance that an ONU uses substantially
the same clock frequency for upstream transmission as the clock
frequency used by the OLT, so that an upstream transmission window
can properly accommodate a data burst. FIG. 3A illustrates an ONU
generating a loop-back clock frequency based on an OLT's downstream
clock frequency (prior art). As is shown in FIG. 3A, according to
the IEEE 802.3ah standards, an ONU 304 can derive its transmission
clock frequency, f, based on a loop-back clock frequency f used by
an OLT 302 for downstream transmission. The loop-back clock is
typically generated by a phase-locking loop (PLL), which is
designed to reduce jitter and track the frequency of the imperfect
recovered clock from the receiver. The loop-back clock frequency
f', however, is still subject to jitter and drifting introduced by
the derivation circuit. Such jitter and drifting may impair of the
quality of upstream transmission.
[0042] In particular, excessive frequency jitter or drifting can
cause errors in upstream transmission. As is shown in FIG. 3A, when
an upstream burst 312 is transmitted with a slower clock frequency,
the time required to finish the transmission may be greater than
the duration of the allocated transmission window. Consequently,
the tail of burst 312 may overlap with a subsequent burst 314,
causing transmission error for both bursts. Hence, it is necessary
to ensure that a transmitted burst remains within the allocated
transmission window.
[0043] To prevent the aforementioned problem, one embodiment of the
present invention uses a separately generated clock frequency at
each ONU. This clock frequency may be generated, for example, with
a local oscillator and ideally is substantially the same as the
clock frequency used by the OLT. A local oscillator is preferred
due to its lower jitter output. Reduced jitter decreases the bit
error rate and improves the quality of the upstream transmission.
FIG. 3B illustrates two ONUs equipped with locally generated clock
frequencies in accordance with an embodiment of the present
invention. As is shown in FIG. 3B, an OLT 322 transmits downstream
data to ONUs 324 and 326 with a clock frequency f. ONU 324
transmits its upstream data at its own clock frequency f'.
Similarly, ONU 326 transmits upstream data at clock frequency
f''.
[0044] Ideally, both f' and f'' are substantially the same as f. It
is difficult, however, to ensure that both f' and f'' are exactly
the same as f. To mitigate the effect of excessive frequency drift,
one embodiment of the present invention allows modification of the
number of bits included in a burst, so that the transmitted burst
remains within the allocated transmission window.
[0045] FIG. 4 illustrates the process of inserting an idle
character into or removing an idle character from an inter-packet
gap (IPG) within a data burst transmitted from an ONU in accordance
with an embodiment of the present invention. As is shown in FIG. 4,
a burst to be transmitted from an ONU typically comprises a lock-in
period 402 (which is used by the OLT to perform bit-level
synchronization with the received data burst), a number of
datagrams or packets (such as datagrams 404 and 408), and
inter-packet gaps (IPG) (such as IPG 406). Note that each datagram
may include a preamble.
[0046] In one embodiment of the present invention, when the ONU's
local clock slows down to the point that the total time required to
transmit the burst is greater than a preferred value by, for
example, one idle character, the ONU may remove one idle character
from an IPG. In this way, the ONU can ensure that the actual
transmission time of the burst remains substantially accurate with
regard to the allocated transmission window.
[0047] Similarly, when the ONU's local clock speeds up to the point
that the total time required to transmit the burst is less than a
preferred value by, for example, one idle character, the ONU may
insert one additional idle character into an IPG. Note that the ONU
can calculate the preferred value of the transmission time based on
the clock frequency used by the OLT for downstream
transmission.
[0048] Another approach to compensate for the clock-frequency drift
at an ONU is to compare the local clock frequency with that of the
OLT and to request a transmission window accordingly to compensate
for the frequency discrepancy. For example, if an ONU's local clock
happens to have slowed down, and the ONU has n bits of data burst
to transmit, the ONU may request a transmission window of if bits
(n'>n) based on the OLT's clock frequency. Consequently; the ONU
can successfully transmit the n-bit burst within the allocated
window, even with a slower clock.
[0049] The foregoing descriptions of embodiments of the present
invention have been presented for purposes of illustration and
description only. They are not intended to be exhaustive or to
limit the present invention to the forms disclosed. Accordingly,
many modifications and variations will be apparent to practitioners
skilled in the art. Additionally, the above disclosure is not
intended to limit the present invention. The scope of the present
invention is defined by the appended claims.
* * * * *